1. Field of the Invention
The present invention relates generally to integrated circuits, and in particular, to a circuit and a method for facilitating the control of bit lines in preparation for, or during, sense amplification of data signals from thinly capacitively-coupled thyristor (“TCCT”)-based memory cells.
2. Description of Related Art
Random Access Memories (“RAM”) are memories capable of multiple read-write cycles and are widely used to temporally store data in computing applications. A typical RAM is structured to include numerous memory cells arranged in an array of rows and columns wherein each memory cell is designed to store a datum or unit of data as a binary digit (i.e., a binary zero or a binary one). Each row of the memory cell array is typically connected to a word line and each column of the memory cell array is typically connected to a bit line (or a pair of complementary bit lines in some memories, such as an SRAM-based memory). The typical RAM structure also includes other circuitry to effect traditional read and write operations, such as reference signal generation circuitry, control signal circuitry, and sensing circuitry.
A constituent memory cell for a RAM is disclosed in U.S. Pat. No. 6,229,161 issued to Nemati et al., which is incorporated herein by reference in its entirety. Netmati et al. discloses capacitively coupled NDR devices for use as SRAM memory cells (also referred herein as TRAM cells). The cells disclosed by Nemati et al. are hereinafter referred to as thinly capacitively coupled thyristor (“TCCT”)-based memory cells wherein the NDR device is a TCCT device. FIG. 1 shows a pair of representative TCCT-based memory cells 10 as disclosed by Nemati et al., and FIG. 2 shows a cross-section through one of the pairs of TCCT-based memory cell 10 along the line 2—2. FIG. 3 shows a schematic circuit diagram corresponding to the TCCT-based memory cell illustrated in FIGS. 1 and 2. The TCCT-based memory cell 10 includes an NDR device 12 and a pass, transistor 14. A charge-plate or gate-like device 16 is disposed adjacent to, and in the case of the illustrated embodiment, surrounding, the NDR device 12. A P+ region 18 of the NDR device 12 is connected to a metallization layer 20 so that a first voltage V1, such as VDDA, can be applied to the NDR device 12 through the P+ region 18. An N+region of the NDR device 12 forms a storage node 22 that is connected to a source of the pass transistor 14.
Successive TCCT-based memory cells 10 are joined by three lines, a bit line 26, a first word line (WL1) 28, and a second word line (WL2) 30. The bit line 26 connects a drain 32 of pass transistor 14 to successive TCCT-based memory cells 10. In a similar fashion, pass transistor 14 includes a gate 34 that forms a portion of the first word line 28. Likewise, the gate-like device 16 forms a portion of the second word line 30.
TCCT-based memory cell 10 has both an “on” state and an “off” state. In the “on” state TCCT-based memory cell 10 generates a current that is received by bit line 26. In the “off” state TCCT-based memory cell 10 produces essentially no current. Second word line 30 is configured to write a state to the TCCT-based memory cell 10, while first word line 28 is generally configured to read the state of the TCCT-based memory cell 10.
An inherent characteristic of TCCT-based memory cell 10 is that the NDR device 12 can develop a leakage current (“ILEAK”) when voltage V1(e.g., VDDA) is applied thereto. ILEAK typically is generated in stand-by mode when some TCCT-based cells are accessed for a read or write operation while other TCCT-based cells are not. When the WL2 line is coupled to both a cell that is being accessed and to a cell that is not, ILEAK will be generated by the non-accessed cell. In particular, when an active WL2 signal is applied to the non-selected NDR device 12, the non-selected pass transistor 14 provides a path from the pass transistor's source terminal to its drain terminal. In this instance, ILEAK flows from NDR device 12 through both node “N” 40 and pass transistor 14 to bit line 26. Generally, one or more circuits connected to bit line 26 provide a path to ground, where such circuits typically can include a bit line pre-charge circuit, a signal sensing circuit, or other like circuitry.
Signal sensing circuits are commonly employed to sense or detect a low-level data signal voltage (or current) from a memory cell and then amplify it to a specific signal level, such as a value of the voltage (or current) representing a logical one (e.g., VDDA) or zero (e.g., ground). A sense amplifier is one such sensing circuit designed to receive the data signal and a reference signal, and thereafter, resolve the data signal into a logical one or zero.
A drawback of using traditional bit line control techniques in conventional sense amplification is that excess charge is not sufficiently removed from the bit lines and sense amplifier nodes to overcome sense amplification errors. Such errors are typical exacerbated by temperature fluctuations, noise, device mismatch and like factors. Since TCCT-based memory cells provide data signals with very small voltages (e.g., 200 mV) for sense amplification, insufficient removal of excess charge can lead to sensing errors.
Another drawback of using traditional bit line control techniques and circuits is that a collective ILEAK in a memory device decreases power efficiency. Memory devices are designed to include one or more memory arrays, each memory array having numerous bit lines. In turn, each bit line can have more than one TCCT-based memory cells coupled to the bit line. For example, each bit line in a single or open bit line memory architecture can have thirty-two to more than a thousand TCCT-based memory cells coupled thereto. As the number of TCCT-based memory cells connected to the bit line increases, ILEAK increases proportionately. Hence, as ILEAK increases, the power consumed also increases. Moreover, as constituent device size (i.e., channel width) of NDR devices of TCCT-based memory cells continue to decrease, the associated ILEAK contribution from each individual memory cell increases. Conventional sensing circuits and bit line control techniques are thus not designed to minimize power consumption by TCCT-based memory cells during, for example, stand-by mode.